Method and inlet control system for controlling a gas flow sample to an evacuated chamber

ABSTRACT

A method and inlet control system for controlling a gas flow sample to an evacuated chamber such as found in a mass spectrometer, is disclosed. The system utilizes a short inlet passage having an effective opening determined by a tapered diamond or steel tip needle adjacent to the inlet passage. The needle is positionally adjusted with respect to the inlet passage by being mounted on a piezoelectric crystal which is flexed by coupling thereto an electric potential derived by sensing the ions at the ionization chamber of a mass spectrometer, for example, and developing therefrom an electric signal indicative of the total pressure within the ionization chamber. The signal coupled to the piezoelectric crystal is preferably a pulse-width modulated signal with the needle maintaining the inlet passage closed except during the time that the piezoelectric crystal is flexed due to a received pulse. A vacuum pump, and a quadrupole filter, both of which are relatively small, are also disclosed, so that a mass spectrometer system, for example, is sufficiently compact so as to be useful, in conjunction with a respiratory valve, for the analysis of respiratory gases. The method for controlling a gas sample flow to a mass spectrometer, for example, comprises providing an inlet for a gas sample into the ionization chamber of the mass spectrometer, monitoring the pressure within the ionization chamber and developing an electrical signal indicative thereof, and utilizing the developed electrical signal to control the flow of gas sample through the inlet to maintain a substantially constant pressure within the ionization chamber.

United States Patent Sodal et al.

[4 1 Dec. 16, 1975 METHOD AND INLET CONTROL SYSTEM FOR CONTROLLING A GASFLOW SAMPLE TO AN EVACUATED CHAMBER [75] Inventors: Ingvar E. Sodal;Lars Hoivik, both of Boulder; Alexander J. Micco; John V. Weil, both ofDenver; Norman W. Baer, Boulder, all of C010.

[73] Assignee: Regents of the University of Colorado, Boulder, Colo.

[22] Filed: Sept. 23, 1974 [21] Appl. No.: 508,452

Related US. Application Data [62] Division of Ser. No. 355,792, April30, 1973, Pat.

[52] US. Cl. 137/487.5

[51] Int. Cl? F16K 31/02 [58] Field of Search 137/4875, 1, 14, 596.16;

, 128/DIG. 17

[56] References Cited UNITED STATES PATENTS 3,465,732 .9/1969 Kattchee137/4875 X 3,747,634 7/1973 Eufusia 137/4875 Primary ExaminerMartin P.Schwadron Assistant ExaminerRobert J. Miller Attorney, Agent, orFirm-Burton, Crandell & Polumbus [57] ABSTRACT A method and inletcontrol system for controlling a BIMORPH CRYSTAL DIAMOND TIP NEEDLE- gasflow sample to an evacuated chamber such as found in a massspectrometer, is disclosed. The system utilizes a short inlet passagehaving an effective opening determined by a tapered diamond or steel tipneedle adjacent to the inlet passage. The needle is positionallyadjusted with respect to the inlet passage by being mounted on apiezoelectric crystal which is flexed by coupling thereto an electricpotential derived by sensing the ions at the ionization chamber of amass spectrometer, for example, and developing therefrom an electricsignal indicative of the total pressure within the ionization chamber.The signal coupled to the piezoelectric crystal is preferably apulse-width modulated signal with the needle maintaining the inletpassage closed except during the time that the piezoelectric crystal isflexed due to a received pulse. A vacuum pump, and a quadrupole filter,both of which are relatively small, are also disclosed, so that a massspectrometer system, for example, is sufficiently compact so as to beuseful, in conjunction with a respiratory valve, for the analysis ofrespiratory gases. The method for controlling a gas sample flow to amass spectrometer, for example, comprises providing an inlet for a gassample into the ionization chamber of the mass spectrometer, monitoringthe pressure within the ionization chamber and developing an electricalsignal indicative thereof, and utilizing the developed electrical signalto control the flow of gas sample through the inlet to maintain asubstantially constant pressure within .the ionization chamber.

8 Claims, 4 Drawing Figures PULSE-WIDTH MODULATED CONTROL SIGNALWin-Roi. AME

7MAss FILTER MULTIPUER FILAMEN'II' TO VACUUM PUMP (42) SIGNAL U.S.Patent Dec. 16, 1975 Sheet20f3 3,926,209

mmhdm MJOmDKnZDQ wmDmmwE U.S. Patent Dec. 16, 1975 Sheet 3 0f 33,926,209

PRESS /o O TORR PRESS W '5 5 x IO I I I I TIME (sec) PRESS %o TORR '5 xIO 0 PRESS 5 5 X IO- --2I I I I I I TIME (sec) METHOD AND INLET CONTROLSYSTEM FOR CONTROLLING A GAS FLOW SAMPLE TO AN EVACUATED CHAMBERCROSS-REFERENCE TO RELATED APPLICATION This is a division of applicationSer. No. 355,792, filed Apr. 30, 1973 now U.S. Pat. No. 3,895,231.

FIELD OF THE INVENTION This invention relates to a method and inletcontrol system for controlling a gas flow sample to an evacuated chamberincluding such evacuated chambers as are found in a mass spectrometerand a sputtering systern.

BACKGROUND OF THE INVENTION Much of pulmonary physiology is based on theanalysis of respiratory gases. Because of its potential as a high speedaccurate gas analyzer, the mass spectrometer has attracted considerableattention in this field. However, the instrument has failed to reach itspotential at least in part due to the necessity for a long capillaryinlet system which can, and often does, destroy the integrity of the gassample and causes instability in the instrument.

Thus, while mass spectrometers have been available to respiratoryphysiologists for about 20 years, they have not achieved the widespreadapplication that was once predicted. With respect to the technicalshortcomings in spectrometer design at least for pulmonary physiologypurposes, the sample inlet system is one of the major problems.

Inherent with mass spectrometry as well as with a sputtering system isthat an immense pressure difference exists between the site at which gasis sampled and the inside of the spectrometer. Traditionally thispressure drop is achieved in two stages. Firstly, a long slendersampling capillary tube is used which produces the major fall inpressure. Secondly, at the end of the capillary a fixed molecular leakis employed to achieve the final pressure drop.

The capillary is required because the size of know spectrometers doesnot permit them to be brought into close proximity to the source ofsample gas such as a respiratory valve. (See Fowler, K.T., TheRespiratory Mass Spectrometer, PHYSICS IN MEDICINE AND BIOLOGY, Volume14, pages 185-199, 1969.) This arrangement has several adverse effectson instrument performance. Firstly, there are distortions introduced bythe behavior of water vapor. During the respiratory cycle sample gasswings between dry inspired and wet expired gas. Water vapor traverses aheated sampling capillary about times more slowly than the otherrespiratory gases. Hence, the ionizer sees a fluctuating water vaporlevel which does not reflect the pressure of water vapor at the frontend of the capillary. Unpredictable errors in precision occur becausethe dilution effect due to water vapor is not the same as existed at themouth. At an oxygen tension of 100 mm Hg this error could be as great as8% if no correction is applied. Various methods for correction of thisproblem have been employed, but only to obtain a more accuraterelationship between the gases of greatest interest. (See Scheid, P.,Slama, H., and Piiper, J., Electronic Compensation of the Effects ofWater Vapor in Respiratory Mass Spectrometry, J. APPL. PHYSIOL, Volume30, pages 258-260, 1971). Secondly, the sampling capillary introducesdelay in response and deterioration of rise time of the instrument.Although this could theo retically be measured and corrected for, smallvariations in pumping speed cause relatively large changes 5 in transittime such that in practice it is difficult to achieve this correctionaccurately. This creates problems when data concerning gas concentrationare to be combined with other information such as gas flow rates as inthe measurement of oxygen uptake. Lastly, even though the geometry ofthe sample conduit and inlet are fixed the actual rate of molecular flowinto the spectrometer tends to vary from moment to moment becausefactors such as particle deposition and changes in gas composition alterthe conductance of the inlet system.

Since the mass spectrometer is a particle counting device variations inmolecular leak rate due to the above factors constitute a source ofrandom error. Hence, it is apparent that the way by which the gas sampleis introduced into the ionizer is the most critical step in themeasurement of respiratory gases by a mass spectrometer. A more accuratemeasurement of the sample line would be made short and the volume of theconduits in front of the ionizer and the ionization chamber madesmaller.

SUMMARY OF THE INVENTION This invention provides an evacuated chamber,such as is found in a mass spectrometer system, that does not require alengthy capillary inlet tube. The inlet system of this inventionincludes valve means adjacent to a small orifice providing an inletpassage into the evacuated chamber of, for example, a mass spectrometer,with the valve means being positionally controlled by valve positioncontrol means that may be made responsive to an electrical signalderived by monitoring the pressure within the evacuated chamber. Byutilizing the foregoing, system stability is improved and accuratemeasurement of all respiratory gases, including water vapor, isfacilitated. In addition, by reducing component size, the overall systemis made sufficiently compact so as to be particularly useful for directattachment to a respiratory valve.

It is therefore an object of this invention to provide a new and novelmethod and inlet control system for controlling a gas flow sample to anevacuated chamber.

It is another object of this invention to provide an improved massspectrometer system that is compact yet provides good stability andaccurate measurements.

It is another object of this invention to provide an improved massspectrometer having a new and novel inlet system.

It is still another object of this invention to provide an inlet systemfor an evacuated chamber that does not require a lengthy capillary inlettube.

It is yet another object of this invention to provide an inlet systemfor an evacuated chamber, including that used in a mass spectrometer,that includes a valve means and valve position control means.

It is another object of this invention to provide a servo-controlledinlet system for an evacuated chamber that automatically maintains thetotal pressure within said chamber at a predetermined level.

It is yet another object of this invention to provide a unique methodfor controlling sample gas flow to a mass spectrometer.

It is still another object of this invention to provide an inlet controlsystem for a mass spectrometer that has a 3 low flow capability withoutadversely affecting good system stability.v

With these and other objects in view, which will be.- come apparent toone skilled in the art as the description proceeds, the inventionresides in the novel construction, combination, and arrangement of partssubstantially as hereinafter described, and more particularly defined bythe appended claims,'.it being understood that such changes in theprecise embodiment of the herein disclosed invention are meant to beincluded as come within the scope of the claims.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings illustrate acomplete embodiment of the invention according to the best mode so fardevised for the practical application of the principles thereof, and inwhich:

FIG. 1 is a simplified schematic representation of the mass spectrometersystem of this invention including a servocontrolled inlet system;

FIG. 2 is a simplified schematic representation of a mass spectrometeras shown in FIG. 1 but showing the sytem attached to a respiratoryvalve;

FIG. 3 is an illustration of a typical cycle during normal operation ofthe system of this invention; and I FIG. 4 is an illustration of atypical cycle when the inlet system of this invention is not utilized.

DESCRIPTION OF THE Preferred EMBODIMENT Referring now to the drawings inwhich like numerals have been used for like characters, the numeral 7refers generally to the mass spectrometer of this invention. As

I shown in FIGS. 1 and 2, the mass spectrometer system includes an inletpassage 8 through which sample gas is introduced into ionization chamber9. The inlet passage 8 is formed in the top portion of the spectrometerhousing 11, which housing may be formed by a thin stainless steelmembrane (6 mills) witha small hole (2 mills) 12 which can be occludedto a varying degree by a diamond-tipped or steel-tipped needle 13. Theneedle is mounted on a piezoelectric crystal 14 (at the end of bodyportions 15 of needle 13- opposite the tapered diamond or steel tip 16),which crystal has the property of flexing when an electric potential isapplied to it (described more fully, for example, in Berlincourt, D.A.,Piezoelectric Transducers, ELECTRO-TECH- NOLOGY, pages 3344, January,1970). The movement of the needle and hence the leak rate is a functionof the voltage applied to the crystal.

A signal proportional to the total pressure in the ionizer (as broughtout more fully hereinafter) provides the input to a control amplifier,or signal processing means, 18 through lead 19 to drive the crystal,permitting a servo-controlled movement of the needle so as to maintainionizer pressure constant. This servo-system controlling the leak (i.e.,introduction of sample gas into the ionization chamber of the massspectrometer) has an extremely rapid response time (1- 2 msec) and it iscapable of operating the leak to compensate for the fastest changes ingas composition, water vapor effects, etc., which in existing systemschange the ionization pressure and thereby introduce errors in thesignal output from the mass spectrometer. As shown schemati cally (at20) in FIG. 1, a pulse-width modulated, pressure-controlled signalcoupled to the crystal from con- With the short distance between theleak and the ionizer both the delay time and the response time aregreatly reduced compared to that of a capillary inlet system.Positioning the crystal (which crystal is conventionally maintained inposition in mount 26, for example) such that when no voltage is applied,the mechanical stress on the crystal is sufficient to close the leak, asindicated in the dotted lines of FIG. 1. Hence, the leak will closeautomatically if the instrument is in a standby mode, or more important,the leak will always be closed if power is removed. This essentiallymeans that the mass spectrometer system is fail safe, and that it can bemoved from one location to another without first going through a complexshut-down procedure. If a high quality vacuum system is employed, it isconceivable that the system could maintain its vacuum over several dayswithout pumping or any form of power input. In a working embodiment ofthis invention, the leak has been tested in a closed position with ahelium leak detector and no measurable leakage was detected. Thistestlwas conducted after the leak had been in continuous operation forthree weeks in a laboratory atmosphere and occasionally exposed toexpired respiratory gas during this period. The same leak was alsotested for its mechanical stability. After being dropped from severalinches down to a table top, the leak was unchanged.

In order to decrease instrument response time without increasing pumpingrequirements, a new smaller and more efficient ionizer is utilized inthis invention. The ionizer has built-in pressure measuring capabilitywhich is used to control the leak rate and for calculation of gastensions. This is achieved by using a small ionization chamber 9 of lessthan 2 cc total volume (see FIG. 1), which conducts a high flow rate ofsample gas directly from the leak, thus providing a fast response timefor the system. A conventional fliament 30 is utilized (electricalconnections are not shown for simplicity) in the ionization chamber. Theion beam is emitted from the chamber through aperture 31. Two plates 33and 34 provide electrostatic focusing of the ion beam and a third plate36 with a smaller orifice 37 picks off part of the ion beam and suppliesthe signal to electrometer 18 for pressure monitoring. The mainadvantage in this unique way of measuring ionizer pressure is that itgives an instant and accurate representation of ionizer pressure as wellas measuring the ion beam which actually enters the mass filter 40 thususing the signal which most directly affects the output of the massspectrometer as a control signal for the leak.

As shown in FIGS. 1 and 2, the sample gas passing through passage 8 isintroduced into ionization chamber 9, and a vacuum pump 42 also isconnected with the ionization chamber, as is common for massspectrometers, through passage 44. After the ion beam is emitted fromionization chamber 9, focused by plates 33 and 34, and a portion pickedoff by plate 36, the beam is directed to a mass filter 40, which asindicated in FIG. 2, can be conventional quadrupole filter (such filtersare discussed, for example, in Pedan, J., The Quadrupole Approach,INDUSTRIAL RESEARCH, pages 50-52, April, 1970; and Wiesendanger,I-I.U.D., Quadrupole Mass Spectrometry, AMERICAN LAB- ORATORY, pages35-43, July, 1970). At the outlet of the mass filte'r, the beam isconventionally directed through multiplier 48 to plate 50 where theoutlet signal is developediand coupled from the system through lead 5 2,As indicated in FIG. 2, this signal, along with a signal from plate 36on lead 19, may be coupled to a computer (not shown) for conventionalprocessing.

As shown in FIG. 2, a respiratory valve 56 can be provided. The passage8 preferably communicates with the middle chamber of such a valve sothat both inspiratory and expiratory gas can be sampled by the massspectrometer. As shown in FIG. 2, the gas from a subject is introducedinto the respiratory valve through tube 58, and a micrometer 60 may alsobe provided. The system of this invention has been built and tested witha quadrupole filter. FIG. 3 shows the output of the instrument whentuned to measure oxygen during an expiratory breathing cycle by blowingacross the leak assembly such that a large amount of water vapor andparticles in the expired air were deposited on the leak. No specialmouth piece or tube was attached to the system and the figure is meantto serve only as an illustration where the servocontrol is operational.The chamber pressure was monitored and displayed below the O tracingusing an ionization gauge. The small change in chamber pressure overthis period caused a change of less than 0.5 percent in the oxygensignal. Since the ionization gauge is also affected by the changing gasconcentration (decreased 0 and increased CO in the chamber, anevaluation of the accuracy should be based on the output signal for eachgas from the mass spectrometer. If the servo-control on the leak isdisabled, the leak clogs up very rapidly as indicated in FIG. 4. Here,the system was exposed to a short burst of expiratory gas (approx, onesec.) and the chamber pressure changed several fold. The change inoxygen signal in this case is mainly due to pressure change in theionizer. Even when exposed to room air only, the leak would clog up veryrapidly from dust particles in the air.

Thus, in operation the mass spectrometer of this invention receives gasthrough chamber 8 and the amount of gas introduced into thie ionizationchamber 9 is controlled by a servo-control system which in cludes apiezoelectric crystal (indicated as a bimorph crystal in FIGS. 1 and 2)that flexes due to application of an electric potential. The electricpotential is generated by a sensing plate 36 in the path of the ion beamwith the electrical output signal from plate 36 being coupled to controlamplifier, or signal processing means, 18. As shown in FIG. 1, theoutput to the piezoelectric crystal is preferably a pulse-widthmodulated signal, such as indicated at 20. Such a signal isconventionally formed and not detailed herein, but rather onlyindicated. In like manner, the typical operation of a mass spectrometer,as well as other details have been left out of this description forsimplicity.

In addition to the reference set out hereinabove, the following may beconsulted for further systems and/or component details: Abrahamsson, S.,The Use of Computers In Low Resolution Mass Spectrometry, SCIENCE TOOLS,Volume 14, pages 29-34, 1967; Beckman Instruments, Inc., MetabolicActivity Gas Analyzer, Technical Report; Brubaker, W. M., A Study of theIntroduction of Ions into the Region of Strong Fields Within Aquadrupole Mass Spectrometer, Final Report NASA-CR-91801, August, 1965-October, 1967; Brubaker, W. M. Theoretical and Experimental Comparisonsof Quadrupole Mass Analyzers with Round and Hyperbolic Field-formingSurfaces, Invited Paper, International Conference on Mass Spectrometry,September, 1969, Kyoto, Japan; Dardik, H., and Laufman, I-I., On-line InVivo Measurements of Partial Pressure of Oxygen and Carbon Dioxide ofBlood, Tissue, and Respired Air by Mass Spectrometry, SURG. GYN. &OBSTET, Volume 131, pages 1157-1160, 1970; Dawson, P. H. Hedman, J. S.,and Whetten, N. R., A Simple Mass Spectrometer, THE REVIEW OF SCIENTIFICINSTRU- MENTS, Volume 40(11), pages 1444-1450, November 1969, Jones, W.B., Finchum, R. N., Russell R. 0. Jr., and Reeves, T. J TransientCardiac Output Response To Multiple Levels of Supine Exercise, J. APPL.PHYSIOL, Volume 28, pages 183-189, 1970; Jones, W. B., Reeves, T. J.,Total Cardiac Output Response During Four Minutes of Exercise, AMER,HEART J., Volume 76, pages 209-216, 1968; and Kim, T. S., Rahn, H. andFarhi, L. E. Estimation of True Venous and Arterial P by Gas Analysis ofa Single Breath, J. APPL. PHYSIOL, Volume 21, pages 1338-1344, 1966.

Although the above description relates to the use of a method and inletcontrol system for controlling a gas flow sample to a mass spectrometerand a novel mass spectrometer, it will be understood that this inventionis not so limited and may be used in controlling pressures in vacuumchambers such as, for example, those used in a sputtering system.Accordingly, it will now be appreciated that this invention relates toan inlet'control system for an evacuated chamber in which said systemcomprises an inlet passage opening into the interior of said evacuatedchamber to introduce gas samples therethrough, valve means at said inletpassage for controlling the effective opening through said passage, andvalve control means for controlling the positioning of said valve meansthereby to control the introduction of gas samples to said evacuatedchamber. The evacuated chamber into which the gas samples are introducedmay be constructed with a relatively small volume, i.e., less than about2'cc total volume. The valve means may be made to be responsive to thepressure within the evacuated chamber thereby controlling the openingthrough the inlet passage to maintain a substantially constant pressurewithin the evacuated cham ber. Further, this invention relates to amethod for controlling gas flow sample to an evacuated chambercomprising providing an inlet for gas sample into the evacuated chamber,sensing or monitoring the pressure within the evacuated chamber anddeveloping or gencrating an electrical signal indicative thereof, andutilizing the generated or developed signal to control the flow of gassample through the inlet to maintain a sub stantially constant pressurewithin the evacuated chamber. The method also includes providing arespiratory valve from which gas sample is taken through said inlet sothat both inspired and expired gas samples may be tested.

From the foregoing, it can be seen that this invention provides a newand novel method and inlet system for an evacuated chamber, as well as anew and novel mass spectrometer.

What is claimed is:

l. A servo-controlled inlet system for an evacuated chamber, said systemcomprising: an inlet passage opening into the interior of an evacuatedchamber and through which gas may be introduced into said chamber; valvemeans at said inlet passage for controlling the effective openingthrough which gas may be introduced into said passage; valve positioningmeans connected with said valve means to position the same; sensingmeans for sensing the pressure within said evacuated chamber anddeveloping an electrical signal indicative thereof; and signalprocessing means including control amplifier means for receiving saidsignal from said sensing means and responsive thereto producing apulse-width modulated control signal that is coupled to said valvepositioning means to control the same whereby said valve means isautomatically adjusted in position dependent upon sensed pressure.

2. The system of claim 1 wherein said valve positioning means includes apiezoelectric crystal connected with said valve means whereby said valvemeans is positionally controlled by the flexing of said crystal.

3. An inlet control system for an evacuated chamber, said systemcomprising: an inlet passage opening into the interior of said evacuatedchamber to introduce gas samples therethrough; valve means including aneedle valve one end of which is tapered and positionally adjustableadjacent to said inlet passage to control the effective opening throughsaid passage; a piezoelectric crystal; means connecting the centerportion of said piezoelectric crystal with said needle valve so that thepositioning of said valve is controlled by the amount of flexing of saidcrystal; sensing means within the evacuated chamber for developing anelectrical signal that is indicative of the pressure within saidevacuated chamber; and signal processing means connected to receive saidelectrical signal from said sensing means and producing an output thatis coupled to said piezoelectric crystal to control the amount offlexing of said crystal.

4. The system of claim 3 wherein said piezoelectric crystal is a bimorphcrystal that maintains said needle valve in a position such that saidinlet passage is closed except when a predetermined signal is receivedfrom said signal processing means to flex said crystal in a manner so asto open said valve.

5. The system of claim 4 wherein said signal processing means produces apulse-width modulated signal that is coupled to said piezoelectriccrystal so that said crystal maintains said inlet passage closed exceptdur- 8 ing the occurrence of eachpulse of said pulse-width modulatedsignal.

6. An evacuated chamber system comprising: an evacuatable chambers; gassampling means including an inlet passage opening into said evacuatablechamber 'to conduct gas samples thereto; valve means for deter miningthe effective opening through said inlet passage; sensing means forsensing the pressure within said evacuatable chamber and developing anelectrical signal indicative thereof; and control means including abimorph piezoelectric crystal the center portion of which is connectedwith said valve means, said control means receiving said signal fromsaid sensing means and responsive thereto controlling the flexing ofsaid piezoelectric crystal to thereby positionally control said valvemeans dependent upon sensed pressure.

7. An evacuated chamber system comprising: an evacuatable chamber; avacuum pump connected with said evacuatable chamber; an inlet passageopening into said evacuatable chamber and through which gas may beintroduced into said evacuatable chamber; valve means including a needlevalve one end of which is tapered and positionally adjustable adjacentto said inlet passage to control the effective opening thereof; apiezoelectric crystal; means connecting the center portion of saidpiezoelectric crystal with said needle valve so that positioning of saidvalve is controlled by the amount of flexing of said crystal; sensingmeans within said evacuatable chamber of sensing the pressure withinsaid chamber and developing an electrical signal indicative thereof; andsignal processing means connected with said sensing means and with saidpiezoelectric crystal whereby a signal developed by said sensing meansis utilized to control the amount of flexing of said piezoelectriccrystal.

8. The system of claim 7 wherein said elements are relatively small sothat said system is sufficiently compact to enable close useage andenhance stability.

1. A servo-controlled inlet system for an evacuated chamber, said systemcomprising: an inlet passage opening into the interior of an evacuatedchamber and through which gas may be introduced into said chamber; valvemeans at said inlet passage for controlling the effective openingthrough which gas may be introduced into said passage; valve positioningmeans connected with said valve means to position the same; sensingmeans for sensing the pressure within said evacuated chamber anddeveloping an electrical signal indicative thereof; and signalprocessing means including control amplifier means for receiving saidsignal from said sensing means and responsive thereto producing apulsewidth modulated control signal that is coupled to said valvepositioning means to control the same whereby said valve means isautomatically adjusted in position dependent upon sensed pressure. 2.The system of claim 1 wherein said valve positioning meanS includes apiezoelectric crystal connected with said valve means whereby said valvemeans is positionally controlled by the flexing of said crystal.
 3. Aninlet control system for an evacuated chamber, said system comprising:an inlet passage opening into the interior of said evacuated chamber tointroduce gas samples therethrough; valve means including a needle valveone end of which is tapered and positionally adjustable adjacent to saidinlet passage to control the effective opening through said passage; apiezoelectric crystal; means connecting the center portion of saidpiezoelectric crystal with said needle valve so that the positioning ofsaid valve is controlled by the amount of flexing of said crystal;sensing means within the evacuated chamber for developing an electricalsignal that is indicative of the pressure within said evacuated chamber;and signal processing means connected to receive said electrical signalfrom said sensing means and producing an output that is coupled to saidpiezoelectric crystal to control the amount of flexing of said crystal.4. The system of claim 3 wherein said piezoelectric crystal is a bimorphcrystal that maintains said needle valve in a position such that saidinlet passage is closed except when a predetermined signal is receivedfrom said signal processing means to flex said crystal in a manner so asto open said valve.
 5. The system of claim 4 wherein said signalprocessing means produces a pulse-width modulated signal that is coupledto said piezoelectric crystal so that said crystal maintains said inletpassage closed except during the occurrence of each pulse of saidpulse-width modulated signal.
 6. An evacuated chamber system comprising:an evacuatable chambers, gas sampling means including an inlet passageopening into said evacuatable chamber to conduct gas samples thereto;valve means for determining the effective opening through said inletpassage; sensing means for sensing the pressure within said evacuatablechamber and developing an electrical signal indicative thereof; andcontrol means including a bimorph piezoelectric crystal the centerportion of which is connected with said valve means, said control meansreceiving said signal from said sensing means and responsive theretocontrolling the flexing of said piezoelectric crystal to therebypositionally control said valve means dependent upon sensed pressure. 7.An evacuated chamber system comprising: an evacuatable chamber; a vacuumpump connected with said evacuatable chamber; an inlet passage openinginto said evacuatable chamber and through which gas may be introducedinto said evacuatable chamber; valve means including a needle valve oneend of which is tapered and positionally adjustable adjacent to saidinlet passage to control the effective opening thereof; a piezoelectriccrystal; means connecting the center portion of said piezoelectriccrystal with said needle valve so that positioning of said valve iscontrolled by the amount of flexing of said crystal; sensing meanswithin said evacuatable chamber of sensing the pressure within saidchamber and developing an electrical signal indicative thereof; andsignal processing means connected with said sensing means and with saidpiezoelectric crystal whereby a signal developed by said sensing meansis utilized to control the amount of flexing of said piezoelectriccrystal.
 8. The system of claim 7 wherein said elements are relativelysmall so that said system is sufficiently compact to enable close useageand enhance stability.